This invention relates generally to telephone signal transmission systems and, more particularly, relates to analytical tools for providing non-conductive detection and identification of digital signal types on lines used for transmitting electrical signals.
DSL (Digital Subscriber Line) technology is a relatively new technology that increases the data transmission rate of ordinary telephone lines substantially compared to common V.34 (33600 bps) or V.90 (56 Kbps) modems. DSL systems are either asymmetric or symmetric. Asymmetric systems provide different transmission rates depending on direction. Asymmetric systems are accordingly well suited for Internet access tasks and video on demand operations. Symmetric DSL provides data transmission at the same rate in both directions. DSL uses a packet switching technology that operates independently of the ordinary voice telephone system. The maximum transmission rate of a DSL system decreases as the distance between transmitting and receiving sites increases. This is predominantly due to voltage deterioration and signal spreading experienced by digital signals as a function of distance.
Often, service personnel or craftspersons wish to determine which line or lines in a given bundle of lines are carrying DSL signal. Furthermore, because conductive probing of DSL lines can damage wires, cause signal deterioration, and present a serious shock hazard due to high line voltages on some wires, non-conductive probing of such lines is desirable. Known DSL probing methods designed to meet these requirements are lacking in other respects. The devices disclosed in U.S. Pat. Nos. 5,297,167 and 5,140,614 capacitively sense voltage changes on a line, and convert the sensed voltage changes to an audible signal. The strength and frequency of the audible signal are used by the technician to surmise whether the probed line is carrying DSL signals. A relatively weak audible signal emitted by the probe may indicate the presence of electromagnetic coupling with nearby lines rather than an actual DSL signal on the probed line.
The probes of the U.S. Pat. Nos. 5,297,167 and 5,140,614 patents both utilize a heterodyning technique. This technique entails combining a capacitively detected high frequency (f1) DSL signal with a locally generated signal of a frequency (f2) that differs by an amount f3, where f3 is audible to the human ear. The resultant signal has both high frequency (f1+f2) and low frequency (|f1xe2x88x92f2|=f3) components. A low pass filter removes the high frequency component leaving an audible signal of frequency f3.
It can be seen that if the frequency of the probed signal differs from that of the local oscillator by more than the range of human hearing, which spans at most 20 kHz, then the filtered heterodyned signal will not be in the human-audible range. This has necessitated the use of multiple probes for multiple carrier types. For example, one probe is required for T1 lines, which operate at 1.544 Mb/s and another for E1 lines, which operate at 2.048 Mb/s.
A digital signal probe is needed that easily, accurately, and non-conductively detects digital signals.
The present invention remedies a number of the shortcomings in prior known hand-held detectors. In accordance with the present invention, a detector containing signal detection and processing circuitry counts at least a portion of the signal transitions on a probed line. The detector further contains threshold circuitry that reduces signal identification errors resulting from weak electromagnetic coupling, or xe2x80x9ccross-talk.xe2x80x9d Because the detector operates by counting transitions rather than heterodyning, there is no similar limitation on the identifiable frequency ranges. For example, a single detector according to the present invention preferably detects different carrier types even when they differ greatly in frequency, such as T1 and E1 carrier types.
Additionally, a detector according to the invention may further comprise a human-readable display usable to display an indication of the counted frequency of the probed line. Such indication may be of the carrier type, capacity, or frequency, or other quantifiable indication related to a detected count.